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IEEE Journal of Quantum Electronics

Two- and Three-Photon Pumped Regular and Random Lasing Behaviors in a Scattering Dye Solution With Suspended Polystyrene Microspheres

Guang S. He

Published Mar 6, 2025• 2 citations
Publisher

Abstract

Regular and random lasing properties in a multiphoton active and highly scattering dye solution containing polystyrene microspheres of 366-nm diameter have been experimentally studied. Using 100-fs laser pulses of wavelength 790 nm and 1300 nm as the two- and three-photon excitation sources, the frequency-upconversion regular lasing of ~610 nm wavelength can be generated along both the forward and backward directions, characterized by high directionality, higher spectral intensity, and remarkable spectral narrowing. In the meantime, the random lasing can also be observed along all other directions, characterized by only spectral narrowing but with much lower spectral brightness. The spectral brightness ratio between the forward regular lasing and the random lasing is ($10^{4}$-$10^{5}$):1. Comparing to the input pump beam of ~3 mm diameter and divergence angle of$\theta _{0}~\approx ~0.9$mrad, the forward lasing beam size and divergence angle were ~7 mm and ~(3-4)$\theta _{0}$, whereas the corresponding parameters were ~5 mm and$\sim 1.5~\theta _{0}$for the backward lasing beam, indicating the phase-conjugation property of the latter.

DOI
10.1109/jqe.2025.3548946

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Regular and random lasing properties in a multiphoton active and highly scattering dye solution containing polystyrene microspheres of 366-nm diameter have been experimentally studied. Using 100-fs laser pulses of wavelength 790 nm and 1300 nm as the two- and three-photon excitation sources, the frequency-upconversion regular lasing of ~610 nm wavelength can be generated along both the forward and backward directions, characterized by high directionality, higher spectral intensity, and remarkable spectral narrowing. In the meantime, the random lasing can also be observed along all other directions, characterized by only spectral narrowing but with much lower spectral brightness. The spectral brightness ratio between the forward regular lasing and the random lasing is (10410^{4}104-10510^{5}105):1. Comparing to the input pump beam of ~3 mm diameter and divergence angle ofθ0 ≈ 0.9\theta _{0}~\approx ~0.9θ0​ ≈ 0.9mrad, the forward lasing beam size and divergence angle were ~7 mm and ~(3-4)θ0\theta _{0}θ0​, whereas the corresponding parameters were ~5 mm and∼1.5 θ0\sim 1.5~\theta _{0}∼1.5 θ0​for the backward lasing beam, indicating the phase-conjugation property of the latter.
Optical phase-conjugation properties of the backward-stimulated radiation from a two-photon-pumped frequency-upconversion lasing medium are studied. The gain medium is a new dye solution pumped with ∼11-ns and ∼1064-nm laser pulses; a highly directional and phase-conjugate backward-stimulated emission at ∼616-nm wavelength could be obtained with a net conversion efficiency of ∼10%. The spectral, temporal, and output–input characteristics of this kind of stimulated emission are presented in detail. The phase-conjugation properties of the backward-stimulated emission have been demonstrated by both the far-field (beam-divergence) measurement and the near-field (image-reconstruction) measurement. The wave-front reconstruction behavior is not perfect, especially under a larger aberration influence.
Optical phase-conjugation properties of the backward stimulated radiation from a one-photon pumped lasing medium have been studied. The gain medium is a dye solution (pyrromethene 597 in ethanol) or the same dye doped poly(methyl methacrylate) (PMMA) rod pumped with /spl sim/7-ns and 532-nm laser pulses. A highly directional and phase-conjugate backward stimulated emission at /spl sim/573 nm wavelength could be obtained with a conversion efficiency of /spl sim/42% by using a 1-cm-long dye solution sample at the concentration level of d/sub 0/=0.000 15 mol/L. The spectral, temporal, output/input, and phase-conjugation properties of this backward stimulated emission are presented in detail. The induced aberration influence, which is much larger than the divergence angle of the input pump beam, can be basically removed by the backward stimulated emission; however, the fidelity of the near-field detail for the backward stimulated emission is relatively poor. A quasi-collinear holographic interaction model and a mathematical analysis are presented to explain the basic experimental results.
A frequency-unshifted and backward stimulated Rayleigh scattering can be produced in a linearly transparent but two-photon absorbing medium. Using a novel two-photon active dye solution as the nonlinear medium pumped by 532-nm and ~10-ns laser pulses, a highly directional backward stimulated scattering at the pump wavelength can be readily observed. The experimental results on spectral structure, spatial and temporal behaviors, and output/input relationship of this new type of stimulated scattering are presented. To explain the observed phenomenon and its experimental behaviors, a physical model of feedback mechanism provided by a two-photon-excitation enhanced Bragg grating inside the scattering medium is proposed. Comparing to other types of stimulated scattering, the stimulated Rayleigh-Bragg scattering exhibits the advantages of no-frequency shift, low threshold, and low requirement for pump spectral line-width.
A frequency-unshifted and backward stimulated scattering can be efficiently generated in one-photon-absorption free but two-photon absorbing materials. Using a number of novel two-photon absorbing dye solutions as the scattering media and nanosecond pulsed laser as the pump beams, a highly directional backward stimulated scattering at the exact pump wavelength can be readily observed once the pump intensity is higher than a certain threshold level. The spectral and spatial structures as well as the temporal behavior and optical phase-conjugation property of this new type of backward stimulated scattering have been experimentally studied. This stimulated scattering phenomenon can be explained by using a model of two-photon-excitation enhanced standing-wave Bragg grating initially formed by the strong forward pump beam and much weaker backward Rayleigh scattering beam; the partial reflection of the pump beam from this grating provides an positive feedback to the initial backward Rayleigh scattering beam without suffering linear attenuation influence. Comparing to other known stimulated (Raman, Brillouin, Rayleigh-wing, and Kerr) scattering effects, the stimulated Rayleigh-Bragg scattering exhibits the advantages of no frequency-shift, low pump threshold, and low spectral linewidth requirement.
We report enhanced emission and gain narrowing in Rhodamine 590 perchlorate dye in an aqueous suspension of polystyrene microspheres. A systematic experimental study of the threshold condition for and the gain narrowing of the stimulated emission over a wide range of dye concentrations and scatterer number densities showed several interesting features, even though the transport mean free path far exceeded the system size. The conventional diffusive-reactive approximation to radiative transfer in an inhomogeneously illuminated random amplifying medium, which is valid for a transport mean-free path much smaller than the system size, is clearly inapplicable here. We propose a new probabilistic approach for the present case of dense, random, weak scatterers involving the otherwise rare and ignorable sub-mean-free-path scatterings, now made effective by the high gain in the medium, which is consistent with experimentally observed features.
The coherent properties of the temporally and spectrally narrowed emission of laser-induced fluorescence of organic dyes hosted inside artificial scattering matrices (random lasers) were investigated. The excitation source was a frequency-doubled 200-fs pulsed laser emitting at 400 nm. Spectral and temporal features were simultaneously recorded with a spectrograph and a streak camera operating in photon-counting mode. Photon-number distributions were thus created. The temporal coherence of the laserlike emission above and below the excitation energy threshold was investigated from the photon-number distribution that was obtained.